package linkedList.两两交换链表中的节点;
//给你一个链表，两两交换其中相邻的节点，并返回交换后链表的头节点。你必须在不修改节点内部的值的情况下完成本题（即，只能进行节点交换）。 
// 示例 1： 
//输入：head = [1,2,3,4]
//输出：[2,1,4,3]
// 示例 2： 
//输入：head = []
//输出：[]
// 示例 3： 
//输入：head = [1]
//输出：[1]
// 提示： 
// 链表中节点的数目在范围 [0, 100] 内 
// 0 <= Node.val <= 100 
// Related Topics 递归 链表 👍 2445 👎 0

class ListNode {
    int val;
    ListNode next;

    ListNode() {
    }

    ListNode(int val) {
        this.val = val;
    }

    ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode dummy = new ListNode(-1);
        ListNode current = dummy;
        ListNode second = head.next;
        while (second != null) {
            ListNode second_next = second.next;
            head.next = second_next;
            second.next = head;
            current.next = second;
            head = second_next;
            if (head == null) {
                break;
            }
            second = head.next;
            current = current.next.next;
        }
        return dummy.next;
    }

    public static void main(String[] args) {
        ListNode head = new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4, new ListNode(5)))));
        Solution solution = new Solution();
        solution.swapPairs(head);
    }
}
